Load Balancing with KEMP and CentOS Backend Services

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Load Balancing with KEMP and CentOS Backend Services

L4 or L7 for Load balancing?

For web services, go the L7 route and route all the traffic through the LB (as a default route) to get the benefits.  With DNS, I chose to use the L4 route.. each with their different configurations.

Testing the KEMP LoadMaster in a config similar to this:

Internet --> Firewalls --> KEMP LM (1.252) --> VIP (1.250) on SVR(s)

Make sure your services include the VIP to bind to.

LoadMaster Sample Config – Layer 7
Service Type: HTTP/HTTPs (example)
Force L7: Yes
Transparency: Yes

You will NOT be able to connect to the VIP from the same subnet as the servers, so keep that in mind if you are testing the VIP and are monitoring availability internally on the same subnet.

Use Direct Server Routing (DSR) to Take a Load Off Your Load Balancer

With Direct Server Routing (DSR), you have to run at L4 and not pass the traffic through the VLM for inspection.  The advantages are the ability to off-load the traffic from having to pass through the LVM in general, but you miss out on the core advantages of the KEMP LoadMaster that are built-in services.

Direct server return (DSR) is a load balancing scheme that allows service requests to come in via the load balancer virtual IP (VIP). The responses are communicated by the back-end servers directly to the client. The load is taken off the load balancer as the return traffic is sent directly to the client from the back-end server, bypassing it entirely. You may want to do this if you have larger files to be served or traffic that doesn’t need to be transformed at all on its way back to the client.

Here’s how it works: Incoming requests are assigned a VIP address on the load balancer itself. Then the load balancer passes the request to the appropriate server while only modifying the destination MAC address to one of the back-end servers.

DSR workflow

You need to be aware of the following when using DSR:

  • Address resolution protocol (ARP) requests for the VIP must be ignored by the back-end servers if the load balancer and back-end servers are on the same subnet. If not, the VIP traffic routing will be bypassed as the back-end server establishes a direct connection with the client.
  • The servers handling the DSR requests must respond to heartbeat requests with their own IP and must respond to requests for content with the load balancer VIP.
  • Application acceleration is not a possibility because the load balancer does not handle the responses from the backend servers.

Here are the configuration steps for Linux (our VIP is and our Physical Server IP is

CentOS Server Configuration

Disable invalid ARP replies by adding the following to the /etc/sysctl.conf file.  Note:  If your interface is eth0, then make the appropriate change below:

Edit /etc/sysctl.conf

net.ipv4.conf.all.arp_ignore = 1
net.ipv4.conf.ens32.arp_ignore = 1
net.ipv4.conf.all.arp_announce = 2
net.ipv4.conf.ens32.arp_announce = 2

Now, reload sysctl values:

# sysctl -p

If using Layer 7 vs. DSR at Layer 4 – Edit the default route to point to the Load Balancer (1.252) vs. the normal default route.  Change the gateway address to the Load Balancer, then restart the network.

# vi /etc/sysconfig/network-scripts/ifcfg-ens32
# systemctl restart network

Add loopbacks for the service once we turn off the ability to respond to ARP requests

 Create an additional loopback interface with an IP alias (the load balancer VIP is represented by, use the ifconfig command:
# ifconfig lo:0 netmask

Enter the following command to verify configuration:

# ifconfig lo:0
lo:0: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet netmask
loop txqueuelen 0 (Local Loopback)

Note that if the machine reboots, this configuration will not be persistent. To set this permanently, some Linux configuration files need to be edited. Steps on how to do this vary from distribution to distribution, but typically you would add the interface to the appropriate loopback in your /etc/sysconfig/network-scripts/ifcfg-lo:X file.

Direct Routing and the ARP Limitation

While there are many advantages to using direct routing in Load Balancer , there are limitations as well. The most common issue with Load Balancer via direct routing is with Address Resolution Protocol (ARP).
In typical situations, a client on the Internet sends a request to an IP address. Network routers typically send requests to their destination by relating IP addresses to a machine’s MAC address with ARP. ARP requests are broadcast to all connected machines on a network, and the machine with the correct IP/MAC address combination receives the packet. The IP/MAC associations are stored in an ARP cache, which is cleared periodically (usually every 15 minutes) and refilled with IP/MAC associations.
The issue with ARP requests in a direct routing Load Balancer setup is that because a client request to an IP address must be associated with a MAC address for the request to be handled, the virtual IP address of the Load Balancer system must also be associated to a MAC as well. However, since both the LVS router and the real servers all have the same VIP, the ARP request will be broadcast to all the machines associated with the VIP. This can cause several problems, such as the VIP being associated directly to one of the real servers and processing requests directly, bypassing the LVS router completely and defeating the purpose of the Load Balancer setup.
To solve this issue, ensure that the incoming requests are always sent to the LVS router rather than one of the real servers. This can be done by either filtering ARP requests or filtering IP packets. ARP filtering can be done using the arptables utility and IP packets can be filtered using iptables or firewalld. The two approaches differ as follows:
  • The ARP filtering method blocks requests reaching the real servers. This prevents ARP from associating VIPs with real servers, leaving the active virtual server to respond with a MAC addresses.
  • The IP packet filtering method permits routing packets to real servers with other IP addresses. This completely sidesteps the ARP problem by not configuring VIPs on real servers in the first place.

Direct Routing Using arptables

In order to configure direct routing using arptables, each real server must have their virtual IP address configured, so they can directly route packets. ARP requests for the VIP are ignored entirely by the real servers, and any ARP packets that might otherwise be sent containing the VIPs are mangled to contain the real server’s IP instead of the VIPs.
Using the arptables method, applications may bind to each individual VIP or port that the real server is servicing. For example, the arptables method allows multiple instances of Apache HTTP Server to be running bound explicitly to different VIPs on the system.
However, using the arptables method, VIPs cannot be configured to start on boot using standard Red Hat Enterprise Linux system configuration tools.
To configure each real server to ignore ARP requests for each virtual IP addresses, perform the following steps:
  1. Create the ARP table entries for each virtual IP address on each real server (the real_ip is the IP the director uses to communicate with the real server; often this is the IP bound to ens32):
    arptables -A INPUT -d <virtual_ip> -j DROP
    arptables -A OUTPUT -s <virtual_ip> -j mangle --mangle-ip-s <real_ip>
    # arptables -A INPUT -d -j DROP
    # arptables -A OUTPUT -s -j mangle --manage-ip-s

    To list all entries:

    # arptables --list -n
    Chain INPUT (policy ACCEPT)
    -j DROP -d 
    Chain OUTPUT (policy ACCEPT)
    -j mangle -s --mangle-ip-s
    Chain FORWARD (policy ACCEPT)

    This will cause the real servers to ignore all ARP requests for the virtual IP addresses, and change any outgoing ARP responses which might otherwise contain the virtual IP so that they contain the real IP of the server instead. The only node that should respond to ARP requests for any of the VIPs is the current active Load Balancer.

  2. Once this has been completed on each real server, save the ARP table entries by typing the following commands on each real server:
    # arptables-save > /etc/sysconfig/arptables
    # systemctl enable arptables.service

    The systemctl enable command will cause the system to reload the arptables configuration on bootup — before the network is started.

  3. Configure the virtual IP address on all real servers using ip addr to create an IP alias. For example:

    ip addr add dev eth0

  4. Configure Keepalived for Direct Routing. This can be done by adding lb_kind DR to the keepalived.conf file.

Direct Routing Using firewalld

You can avoid the ARP issue using the direct routing method by creating firewall rules using firewalld. To configure direct routing, add rules that create a transparent proxy so that a real server will service packets sent to the VIP address, even though the VIP address does not exist on the system.
The direct routing method is simpler to configure than the arptables method. This method also circumvents the ARP issue entirely, because virtual IP addresses only exist on the active Load Balancer.
However, there are performance issues using the direct routing method compared to arptables, as there is overhead in forwarding, with IP masquerading, every return packet.
You also cannot reuse ports using the direct routing method. For example, it is not possible to run two separate Apache HTTP Server services bound to port 80, because both must bind to INADDR_ANYinstead of the virtual IP addresses.
To configure direct routing for IPv4 using firewalld, perform the following steps on every real server:
  1. Enable IP masquerading for the zone of the network interface that receives the packets from the LVS. For example, for the external zone, as root:

    ~]# firewall-cmd --zone=public--add-masquerade --permanent

    If zone is omitted, the default zone is used. The --permanent option makes the setting persistent, but the command will only take effect at next system start. If required to make the setting take effect immediately, repeat the command omitting the --permanent option.

  2. Enter commands in the following format for every VIP, port, and protocol (TCP or UDP) combination intended to be serviced by the real server:
    firewall-cmd --zone=zone --add-forward-port=port=port_number:proto=protocol:toport=port_number:toaddr=virtual_IP_address

    For example, to configure TCP traffic on port 80 to be redirected to port 3753 at

    # firewall-cmd --zone=external --add-forward-port=port=80:proto=tcp:toport=3753:toaddr= --permanent

    The --permanent option makes the setting persistent, but the command will only take effect at next system start. If required to make the setting take effect immediately, repeat the command omitting the --permanent option.

    This command will cause the real servers to process packets destined for the VIP and port that they are given.
  3. If required, to ensure firewalld is running:

    # systemctl start firewalld

    To ensure firewalld is enabled to start at system start:

    # systemctl enable firewalld

See also Red Hat Load Balancer Administration